1. Field of the Invention
The present invention relates to passivated semiconductor devices such as SiC devices and to methods for fabricating same.
More particularly, the present invention relates to passivated semiconductor devices such as SiC devices, wherein the semiconductor device has treated area(s) or edge(s) in the semiconductor aligned with the edge of an electrical contact layer, and to methods for fabricating and using same.
2. Description of the Related Art
Power devices are widely used to carry large currents and support high voltages. Since the early 1950""s, developers of electronic power systems began to base their power systems on semiconductor devices. Presently, many types of power semiconductor devices are available including, but not limited to, power rectifiers, power bipolar transistors, power field effect transistors, power bipolar/field effect devices, power thyristors and other two, three or more terminal semiconductor devices.
Most power semiconductor devices being marketed today are fabricated in monocrystalline silicon. However, as is known to those skilled in the art, monocrystalline silicon carbide is particularly well suited for use in semiconductor devices and, in particular, for power semiconductor devices. Silicon carbide has a wide energy bandgap, high melting point, low dielectric constant, high breakdown-field strength, high thermal conductivity, and high saturation electron drift velocity compared to silicon. These characteristics would allow silicon carbide power devices to operate at higher temperatures, higher power levels, and with lower specific on-resistance than conventional silicon based power devices. Such devices must also exhibit low reverse leakage currents. Large reverse leakage currents cause premature soft breakdown.
Schottky barrier diodes with a breakdown voltage of approximately 1000 V, a low forward voltage drop of approximately 1.3 V at 100 A/cm2, and reverse leakage current of approximately 0.1 mA/cm2 at 1000 V have been produced from 4H-SiC by a technique which included forming the Schottky contact layer on a specially prepared surface of SiC. Such diodes were unpassivated [1].
Surface passivation is needed for any semiconductor device to protect it from its external environment and for obtaining stable and repeatable electrical characteristics. Without surface passivation, the characteristics of devices such as diodes or rectifiers can be easily influenced by ambient moisture and impurities and thus become unstable.
Schottky barrier diodes, passivated using a variety of dielectrics (e.g., thermally grown or deposited oxides), have forward current voltage characteristics substantially the same as those of unpassivated devices, but have leakage currents orders of magnitude higher than above-mentioned unpassivated devices. The increase in leakage current is thought to be due to the presence of positive charges in the passivation dielectric, which causes the electric field crowding near the edge of the diode.
It is known that large leakage currents in the area surrounding the edges of SiC devices can be reduced by an appropriate edge termination treatment at the periphery of the active contact or rectifying junction. One edge-termination technique, particularly suited to SiC diodes, has been taught by Baliga and Alok [2] and by Alok and Baliga [3]. The technique, which involves ion implantation at the edge of the rectifying junction, improves the breakdown voltage of unpassivated Schottky barrier diodes over that of similar devices without such edge termination [2, 3].
A method of fabricating SiC devices with an ion implanted edge termination and oxide passivation was described by Bhatnagar. et.al. [4]. According to their teaching, the passivating oxide is first deposited over, and then removed from, the area of the device where the rectifying contact is eventually formed. Since it is difficult or impossible to remove all traces of the chemical substances that have come in contact with the semiconductor surface during such processing, the diodes are likely to exhibit sub-optimal electrical characteristics.
It is therefore an object of this invention to provide passivated silicon carbide devices with improved electrical characteristics.
It is another object of the invention to provide a passivated SiC semiconductor device with low reverse leakage currents.
It is still another object of the invention to provide a passivated silicon carbide device with near theoretical breakdown voltages and low reverse leakage currents.
It is another object of the invention to provide a method of fabricating such devices.
In the present invention, ion-implanted (hereafter referred to as ion-treated) edge termination was found to reduce the parasitic leakage current due to passivation of the Schottky barrier diode and, at the same time, maintain or actually improve the breakdown voltage of such diodes relative to unpassivated, unterminated Schottky barrier diodes.
According to the present invention, a SiC semiconductor device is characterized by the combination of an electrical contact layer, formed on an ultra-clean surface of a monocrystalline SiC semiconductor layer, the surface pretreated as described herein, and which has received no additional exposure to other chemical treatments or materials, an ion-treated area aligned with an edge of the contact, and a passivation layer extending from a position at or near the edge of the junction and covering all or a portion of the surface. The electrical contact layer may be a rectifying (Schottky) contact or a contact for a rectifying junction. The device can also include an Ohmic-contact layer on a second surface of the device.
According to a preferred embodiment of the invention, the ion-treated area extends out a distance D of at least 5 xcexcm from the edges of the contact layer.
The present invention also provides a method for fabricating the devices of this invention. The method includes the steps of:
(1) forming a patterned electrical contact layer on a pretreated portion of a first surface of a semiconductor layer; (2) ion-treating an area of the layer aligned with an edge of the contact layer; (3) forming a patterned passivation layer on and covering the first surface of the layer; and (4) forming openings in the passivation layer exposing a portion of the electrical contact layer.
The present invention provides semiconductor devices with near-theoretical breakdown voltages and low leakage currents. It is believed that these superior characteristics are due in part to the formation of the electrical contact layer on a specially prepared surface of monocrystalline SiC, as described herein, and which has received no additional exposure to other chemical treatments (hereafter referred to as ultra-clean surface).